Although the idea of an engine swap sounds great, it's going to get out of hand when every car gets an [insert alphanumeric code here] engine in it. Not to mention, what about making the most of the engine's ability before demanding a 300% increase in power? If you're going to swap out motors for a 20-50hp increase, what's the point? Pick a turbo, or pick an NA kit and keep it simple. There's also going to be lots of arbitrary choices on PD's part.
By the way, PD included a tuning option in all the GT games called "Increase Displacement", but for some reason, they never seem to employ it on more than a dozen or so cars...
In short, it's a great idea that works in real-life, but not video games. I never thought I'd say that.